Vista - shell32.dll not found - Vista Home Premium won't start!

I have a PC to sort out which has the following issue:

On starting up, and before the Windows logo displays it throws up an error
message:
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Title bar text: LogonUI.exe
This application has failed to start because SHELL32.dll was not found.
RE-installing the application may fix the problem.
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Clicking on [OK] then displays the Windows logo, followed by the coloured
background with 'Windows Vista Home Premium' at the bottom, following which
on a black screen I get two error messages. The first is the same as that
above except that the file quoted in the title bar is Explorer.exe. The
second error message appearing behind this one advises that the 'Task
Schedulaer Engine' has stopped responding, clicking [Close program] on this
(and further) error messages I get explorere.exe has stopped working, and
Windows Problem Reporting and stopped working.

Starting in Safe Mode I get to the black and empty desktop with the Safe
Mode labels in the corners but nothing appears to be happening. Entering
'Task Manager' via Ctrl-Alt-Del again gives the same sort of error but this
time the Title Bar of the error box (referring to SHELL32.dll) is headed
Taskmgr.exe.

I have tried the Vista Startup Repair but that had no effect.

SHELL32.DLL is the same size and data and time in System32 as it is in
Dllcache.

Any ideas how to sort this out? Is it a re-install? The machine is an HP
and has a recovery partition.

An early replay appreciated. TIA